You will lose the old data plan it does not matter what phone you upgrade too. I tried 4S, Razr M and Iphone 5, My guess its the 4G LTE and They want to get rid of unlimited plans for everyone. However i changed to share everything (i have 2 lines) with 6GB data and it cost about the same with my 15% discount. Plus i now have unlimited talk.
Iphone 5 on verizon will not surf and chat on the Data network (4g/3g etc) but when your phone is using wifi you will be able to use that feature
The answers to all of your questions are in several other threads that have already been posted to this forum, if you wanna do a search. In a nutshell ...
The only way to keep your unlimited data plan is to pay full retail price for a phone (regardless of which phone you buy) or purchase it from someplace like ebay or craigs list. If you sign a new two-year agreement (i.e., get a subsidized phone), you'll either have to choose a tiered data plan (i.e., $30 for 2 gb) if you want to keep your current calling plan, or switch the entire account to Share Everything. This was put into effect the end of June when the Share Everything plan was introduced.
And if you haven't already heard, as of this spring, there is a $30 upgrade fee.
Several posts on this forum (and several articles on the internet) indicate that the iphone 5 cannot do voice/data at the same time on Verizon. It's not due to the SIM card, but the antennas/chips.